1. Field of the Invention
This invention relates to a metallic mold for casting a vehicle wheel having openings in the periphery of a disk portion thereof.
2. Description of the Related Art
As disclosed in Japanese Published Unexamined Patent Application No. 151344/1990, there is conventionally known a metallic mold which comprises a lower mold, an upper mold and split molds for enclosing the lower and upper molds and in which molten aluminum is filled into a cavity inside the metallic mold by utilizing a gas pressure such as a pneumatic pressure or the like to cast a vehicle wheel by low pressure die casting. In this mold, it is presumed that an external surface which faces the outside when the wheel is actually mounted on a vehicle is formed by the lower mold, that an internal surface of the wheel is formed by the upper mold, and that an outer periphery of a rim portion of the wheel is formed by the split molds, respectively. In order to prevent the external surface of the wheel from giving rise to defects by shrinkage cavities or the like, it is so arranged that the lower mold is water-cooled so that the molten aluminum is solidified from the side of the lower mold, i.e., from the side of the external surface of the wheel.
In addition, it is known, in order to improve the releasing or parting property of the molds and the fluidity of the molten material, to coat the forming surface of each of the molds with a mold coating agent consisting of ceramics.
In case where projections which are used for forming openings and which abut the upper mold are provided on the lower mold in order to form openings such as wind openings which are indented inwards from the external surface in the periphery of the disk portion of the wheel, if the upper mold is cooled later than the lower mold so that the molten material is solidified from the side of the lower mold, the projections trying to be displaced diametrically inwards of the wheel through the shrinkage of the lower mold due to cooling thereof are restricted in their displacement by the upper mold which abuts the projections. As a result, those base portions of the projections which extend diametrically inwards are scratched by the shrinking molten material that shrinks due to solidification, and the mold coating agent of those portions is likely to be peeled off. This results in an increased coating frequency of the mold coating agent and a consequent poor productivity.